1. Field of the Invention
The present invention relates to a surface-acoustic-wave device that utilizes surface acoustic waves propagating along the surface of a solid.
2. Description of the Background Art
Surface-acoustic-wave devices that utilize surface acoustic waves (hereinafter called SAWs) have various features such as small size, light weight, high resistance to vibration and impact, high reliability, high temperature stability, long life, and superior phase properties and hence can be widely used as frequency filters, resonators, delay devices, signal processing elements, convolvers, and functional elements for opto-electronics.
Properties of SAW devices rely largely on the property of the substrate material. The propagation velocity (v) and electromechanical coupling coefficient (K2) properties of a SAW device are particularly important. A higher propagation velocity facilitates an increase in operating frequency, and an increase in electromechanical coupling coefficient can decrease losses over broad bands.
Development has been proceeding on substrate materials having a great propagation velocity and electromechanical coupling coefficient. Such substrate materials include a ZnO/diamond substrate disclosed in published Japanese patent application Tokukaihei 3-198412, a c-axis-oriented multicrystal LiNbO3/diamond substrate disclosed in Tokukaihei 8-32398, and a single-crystal LiNbO3/diamond substrate disclosed in Tokukaihei9-219632.
Of these, Tokukaihei 9-219632 discloses a means to determine the layer thickness and crystal orientation of a single-crystal LiNbO3 layer that achieves such superior properties as v=8,000 to 11,500 m/s accompanied by K2=10 to 20%.
However, it is difficult to further increase an electromechanical coupling coefficient while maintaining a propagation velocity as high as 5,000 m/s or more.
An object of the present invention is to offer a SAW device having not only an increased propagation velocity of SAWs but also an increased electromechanical coupling coefficient of 20% or more. The SAW device of the present invention is used within the frequency range of several hundred megahertz to about 20 GHz.
The present inventors found through intensive studies that the above-mentioned object can be achieved by the following means: For a SAW device in which a single crystal of KNbO3, a niobate similar to LiNbO3, is laminated on a diamond layer and is provided with interdigital transducers (hereinafter called IDTs), the crystal orientation and layer thickness of the single-crystal KNbO3 are controlled so as to increase both the propagation velocity and electromechanical coupling coefficient for the SAW in a mode to be utilized. Subsequently, the present inventors found a crystal orientation and layer thickness of the single-crystal KNbO3 that can fulfill both v=5,000 m/s or more and K2=20% or more at the same time for the SAW in a mode to be utilized.
The present invention is based on the above-mentioned finding. The IDTs may be provided either on the single-crystal KNbO3 layer or between the single-crystal KNbO3 layer and the diamond layer. Another structure may also be adopted in which the IDTs are provided on the single-crystal KNbO3 layer and a short-circuiting electrode is provided between the single-crystal KNbO3 layer and the diamond layer. Each structure has its own desirable crystal orientation and layer thickness of the single-crystal KNbO3. Their desirable ranges are described in the section xe2x80x9cdescription of the preferred embodiments.xe2x80x9d